Miniature mass spectrometers have application as portable devices for the detection of biological and chemical warfare agents, drugs, explosives and pollutants, as instruments for space exploration, and as residual gas analysers.
Mass spectrometers consist of three main subsystems: an ion source, an ion filter, and an ion counter. One of the most successful variants is the quadrupole mass spectrometer, which uses a quadrupole electrostatic lens as a mass filter. Conventional quadrupole lenses consist of four cylindrical electrodes, which are mounted accurately parallel and with their centre-to-centre spacing at a well-defined ratio to their diameter [Batey 1987].
Ions are injected into the pupil between the electrodes, and travel parallel to the electrodes under the influence of a time-varying hyperbolic electrostatic field. This field contains both a direct current (DC) and an alternating current (AC) component. The frequency of the AC component is fixed, and the ratio of the DC voltage to the AC voltage is also fixed.
Studies of the dynamics of an ion in such a field have shown that only ions of a particular charge to mass ratio will transit the quadrupole without discharging against one of the rods. Consequently, the device acts as a mass filter. The ions that successfully exit the filter may be detected. If the DC and AC voltages are ramped together, the detected signal is a spectrum of the different masses that are present in the ion flux. The largest mass that can be detected is determined from the largest voltage that can be applied.
The resolution of a quadrupole filter is determined by two main factors: the number of cycles of alternating voltage experienced by each ion, and the accuracy with which the desired field is created. So that each ion experiences a large enough number of cycles, the ions are injected with a small axial velocity, and a radio frequency (RF) AC component is used. This frequency must be increased as the length of the filter is reduced.
The sensitivity and hence the overall performance of a mass spectrometer is also affected by the signal level and the noise level. Noise arising from stray ions is conventionally reduced by the use of a grounded screen [Denison 1971]. The ion transmission is clearly reduced as the size of the entrance pupil is decreased. Efforts have therefore been made to improve transmission in small quadrupoles, and it has been shown that significantly improved transmission at a given resolution can be obtained by reducing the effect of fringing fields at the input to the quadrupole.
One effective method involves the use of a so-called Brubaker lens or Brubaker pre-filter, which consists of an additional set of four short, cylindrical electrodes mounted co-linearly with the main quadrupole electrodes. The Brubaker pre-filter is excited with the AC voltages (but not the DC voltages) applied to the main quadrupole lens. It is well known that a quadrupole excited only with AC voltages acts as an all-pass filter, so that the Brubaker pre-filter provides an ion guide into the main quadrupole. However, the delay in application of the DC voltage component results in a reduction in fringing fields and significantly improves overall ion transmission at a given mass resolution [Brubaker 1968; U.S. Pat. No. 3,129,327; U.S. Pat. No. 3,371,204].
In order to create the desired hyperbolic field, highly accurate methods of construction are employed. However, it becomes increasingly difficult to obtain the required precision as the size of the structure is reduced [Batey 1987]. Microfabrication methods are therefore increasingly being employed to miniaturise mass spectrometers, both to reduce costs and allow portability.
Microfabricated devices are often fabricated on silicon wafers, because of the range of compatible deposition, patterning and etching processes that may be used. However, the resistivity of silicon is inherently limited to that of intrinsic material, and the thickness of deposited insulating films is limited by the stress in such films. These restrictions have particular consequences for the performance of RF devices such as electrostatic quadrupole mass filters formed in silicon.
For example, a silicon-based quadrupole electrostatic mass filter consisting of four cylindrical electrodes mounted in pairs on two oxidised, silicon substrates was demonstrated some years ago. The substrates were held apart by two cylindrical insulating spacers, and V-shaped grooves formed by anisotropic wet chemical etching were used to locate the electrodes and the spacers. The electrodes were metal-coated glass rods soldered to metal films deposited in the grooves. [U.S. Pat. No. 6,025,591].
Mass filtering was demonstrated using devices with electrodes of 0.5 mm diameter and 30 mm length [Syms et al. 1996; Syms et al. 1998; Taylor et al. 1999]. However, the performance was limited by RF heating, caused by capacitative coupling between co-planar cylindrical electrodes through the oxide interlayer via the substrate. As a result, the device presented a poor electrical load, and the solder attaching the electrodes tended to melt. These effects restricted the voltage and frequency that could be applied, which in turn limited both the mass range (to around 100 atomic mass units) and the mass resolution. While the substrate was grounded, the use of an incomplete screen also resulted in high noise levels, and the devices also suffered in low transmission rates.
In an effort to overcome these limitations, an alternative construction based on bonded silicon-on-insulator (BSOI) was developed [GB 2391694]. BSOI consists of an oxidised silicon wafer, to which a second silicon wafer has been bonded. The second wafer may be polished back to the desired thickness, to leave a silicon-oxide-silicon multi-layer.
In this geometry, the electrode rods were again mounted in pairs on two substrates. However, the electrodes were now retained by silicon springs etched into the substrate of the BSOI wafer, while the device layer was used as a spacer. The oxide interlayer was largely removed, so that capacitative coupling between co-planar cylindrical electrodes via the substrate was greatly reduced. As a result, the device could withstand considerably higher voltages, and a mass range of 400 atomic mass units was demonstrated [Geear et al. 2005].
Despite these results, only partial screening was again possible. Furthermore, it was found that the transmission was again low, because of obstruction of the entrance pupil by the features such as springs and hooks mounting the cylindrical electrodes. These features also hampered the incorporation of auxiliary optics such as a Brubaker pre-filter.
A further microfabricated quadrupole filter, described as a “square rods quadrupole” and based on a two-substrate assembly formed in silicon and mounting a set of polygonal rods, has also been described [Sillon and Baptist 2002; U.S. Pat. No. 6,465,792]. However, it does not appear to have been demonstrated.
Because many applications of mass spectrometry require greater mass range, there is a need to provide a more effective solution to the problem of RF heating. There is therefore a need to provide such a solution and also a requirement for mass spectrometer devices that are operable in conditions requiring low noise and greater sensitivity at a given resolution.